1. Technical Field of the Invention
The present invention relates generally to spark plugs for internal combustion engines. More particularly, the invention relates to an improved structure of a spark plug for an internal combustion engine of an automotive vehicle which ensures a high capability of the spark plug to ignite the air-fuel mixture (referred to as ignition capability of the spark plug hereinafter).
2. Description of the Related Art
Conventional spark plugs for use in internal combustion engines generally include a metal shell, an insulator, a center electrode, and a ground electrode.
The metal shell has a threaded portion for fitting the spark plug into a combustion chamber of the engine. The insulator has a center bore formed therein, and is fixed in the metal shell such that an end thereof protrudes from an end of the metal shell. The center electrode is secured in the center bore of the insulator such that an end thereof protrudes from the end of the insulator. The ground electrode has a side surface, and is joined to the end of the metal shell such that the side surface thereof is opposed to and spaced from the end of the center electrode so as to form a spark gap therebetween.
In recent years, the demand for higher power output of an internal combustion engine has required increasing the sizes of intake and exhaust valves for the engine and securing a water jacket for cooling of the engine. This results in a decreased space available for installing a spark plug in the engine, thus requiring the spark plug to be slenderized.
For example, the threaded portion of the metal shell of a spark plug had an outer diameter of M14 as specified in JIS (Japanese Industrial Standards) in the past; however, the threaded portion is now required to have an outer diameter of equal to or less than M12 as specified in JIS.
Moreover, the engine types of high compression or lean burn have recently been used in engine design for the purpose of increasing power output or improving fuel economy. When the combustion condition of such a type engine comes to worsen, carbon and other unburned products will deposit on the surface of the insulator around the end thereof. Such deposit causes a problem of “carbon fouling”.
In a slenderized spark plug, the volume of an air pocket is accordingly reduced which is the space between an outer surface of the insulator and an inner surface of the metal shell. The reduced volume of the air pocket can cause generation of “surface-creeping sparks” which move from the center electrode of the spark plug along an outer surface of the insulator, and fly to the metal shell of the spark plug.
Such surface-creeping sparks are more frequently generated in a spark plug where the insulator thereof is fouled with carbon, since the electrically conductive carbon deposit on the surface of the insulator reduces an insulation resistance between the insulator and the metal shell.
To suppress generation of surface-creeping sparks, U.S. Pat. No. 6,147,441 (referred to as a first reference hereinafter) discloses a spark plug which has the threaded portion of a metal shell with an outer diameter in the range of 10–12 mm. The spark plug has specified ranges for dimensional parameters, such as a length of a discharge gap (i.e., a spark gap size), a width of a gas volume (i.e., an air pocket size), a protruding length of an insulator with respect to a fitting piece (i.e., a metal shell), a diameter of a center electrode, an end diameter of a noble metal tip (i.e., noble metal chip), and a protruding height of the noble metal tip with respect to the center electrode.
Moreover, to solve the above-described problem of carbon fouling, U.S. Pat. No. 5,929,556 (referred to as a second reference hereinafter) discloses another type of spark plug. The spark plug has a structure where a center electrode retracts from an end of an insulator, so that, when the insulator is fouled with carbon, the carbon deposit on the surface of the insulator can be burned off during generation of surface-creeping sparks.
The inventors of the present invention have found through investigation that, in a slenderized spark plug that has the structure disclosed in the first reference, the generation of surface-creeping sparks cannot be effectively suppressed even when the insulator thereof is not fouled with carbon.
FIG. 11 shows a spark gap 50 and its proximity in a typical spark plug. The spark plug includes, as shown in the figure, a metal shell 10, and insulator 20, a center electrode 30, and a ground electrode 40. Dimensional parameters, which are employed in the investigation of the inventors for the spark plug disclosed in first reference, are also designated in FIG. 11. Those parameters include:
a clearance X between the center electrode 30 and the insulator 20;
a surface-creeping distance Y of the insulator 20 outside the metal shell 10;
a protruding length Y1 of the insulator 20;
an air pocket size Z;
a spark gap size G; and
a surface-creeping distance W of the insulator 20 inside the metal shell 10.
In addition, a surface-creeping spark distance of the spark plug is represented by a combinational parameter (X+Y+Z).
The relationship between the spark gap size G and the surface-creeping spark distance (X+Y+Z) has a great influence on generation of surface-creeping sparks. More specifically, for a given spark gap size G, a greater surface-creeping spark distance (X+Y+Z) is more advantageous to suppressing generation of surface-creeping sparks.
However, when the structure disclosed in the first reference is applied to a slenderized spark plug, especially to one which has the threaded portion of a metal shell with an outer diameter equal to or less than 10 mm, the air pocket size Z of the spark plug cannot be allowed to have a large value. As a result, the surface-creeping spark distance (X+Y+Z) of the spark plug becomes so small with respect to the spark gap size G that generation of surface-creeping sparks in the spark plug cannot be effectively suppressed.
The spark plug disclosed in the second reference is designed, as described above, to prevent decrease of the insulation resistance between the insulator and the metal shell through burning off the carbon deposit on the insulator surface during generation of surface-creeping sparks, when the insulator is fouled with carbon.
However, the problem of carbon fouling has become very serious to a recent spark plug used in an engine of high compression or lean burn type. A large amount of carbon deposit builds up on the surface of the insulator around the end of the same, so that the insulation resistance of the portion of the insulator protruding from the end of the metal shell comes to decrease, resulting in a short circuit of the spark plug.
Specifically, in FIG. 11, a large amount of carbon deposit builds up in the clearance X between the center electrode 30 and the insulator 20 and on the outer surface of the insulator 20 corresponding to the surface-creeping distance Y, resulting in the short circuit.
When the carbon deposit builds up gradually, it is possible for the spark plug disclosed in the second reference to prevent the decrease of the insulation resistance through burning off the carbon deposit. However, when a large amount of carbon deposit builds up rapidly, the carbon deposit cannot be timely cleaned through burning off.
Further, the inventors of the present invention have found through an investigation that the ignition capability of the spark plug disclosed in the second reference will drop rapidly when surface-creeping sparks are generated in the spark plug.
The investigation has found that the air pocket size Z in the spark plug has a great influence on the ignition capability of the spark plug when surface-creeping sparks are generated in the spark plug. As the air pocket size Z increases, the ignition capability of the spark plug increases.
More specifically, when the surface-creeping sparks are generated in the air pocket of the spark plug, the space for ignition in the air pocket increases as the air pocket size Z increases, thereby facilitating ignition therein. On the contrary, a decrease in the air pocket size Z results in a decrease in the space for ignition, which leads to a misfire of the engine.
The spark plug disclosed in the second reference is, in fact, designed to keep the insulation resistance; however, the ignition capability of the spark plug is not considered under the condition where the surface-creeping sparks are generated in the spark plug.